In the process of fabricating packaged semiconductor devices, adhesive and eutectic die attach methods are two of the most commonly used techniques for attaching semiconductor die to a substrate (e.g., a leadframe, flange, or other substrate). Adhesive die attach methods typically use non-conductive adhesives (e.g., polymer adhesives or epoxies) as die attach material to mount the semiconductor die to the substrate.
In contrast, standard eutectic die attach methods typically use preforms or pastes of conductive eutectic alloys to bond the semiconductor die to the substrate. For example, commonly used die attach materials for standard eutectic die attach processes include pure gold (Au), gold-containing alloys (e.g., gold-tin (Au—Sn)), and lead-containing alloys (e.g., lead-silver-indium (Pb—Ag—In), lead-silver-tin (Pb—Ag—Sn), and lead-tin (Pb—Sn)). When pure gold is used to attach silicon (Si) or germanium (Ge) die to a substrate, for example, the silicon or germanium from the die diffuses into the gold during an initial heating process, forming gold-silicon (Au—Si) or gold-germanium (Au—Ge) eutectic alloys, respectively.
A significant disadvantage to using gold as a die attach material for a eutectic die attach process is the relatively high cost of gold when compared with other materials. However, potential environmental issues and waste disposal costs associated with using lead-containing die attach materials also is a significant factor to consider in determining which eutectic die attach material to use.
Die attach methods also may be classified as pressurized or pressureless methods. Using a pressurized die attach method, after placing the die over the die attach material and the substrate, special equipment is used to press against the top surface of the die in order to compress the die toward the substrate during the heating process. More specifically, the special equipment used to compress the die toward the substrate may include specially machined or formed solids (e.g., metals, ceramics and/or polymers) for each configuration of dies and substrates. In contrast, pressureless die attach methods do not use such special, pressure-applying equipment, and the device experiences only ambient pressure during the die attach process.
In general, pressureless die attach methods tend to yield devices with larger and/or more plentiful voids in the die attach material. These voids may significantly affect the robustness, performance, and reliability of the packaged device. More specifically, large voids in the die attach material may result in relatively low thermal and/or electrical conductivity between the die and the substrate, and/or may yield assemblies with low die shear strength. In addition, die may be more prone to cracking when the die attach material includes relatively large voids. In contrast, pressurized die attach methods may yield devices with relatively small voids in the die attach material. However, the mechanical stresses imparted upon the die during a pressurized die attach process are more likely to lead to die cracking, thus potentially decreasing manufacturing yields.